The advance of the XRD technique allows us to reach the properties of each coarse grain. This paper has demonstrated a new method to determine stress in a single crystal for multicrystal material and this new method could be specially applied for any symmetric crystalline systems. The strain tensor ε is determined by the change of the metric tensor G before the initial state and after the deformed state in the crystal reference system. Then stress tensor at grain scale is calculated by the Hooks law. The stress evaluations are carried out in coarse grains of a thin galvanized coating on a steel substrate during tensile loading. This study allows us to link the microstructure evolution to the elastic heterogeneity at grain scale or between the grains.